1. Field of the Invention
The present invention relates to a digital filter system which cancels unwanted signal components at the intermediate frequency stage.
The present invention also relates to a demodulation circuit for radio communication, more particularly, to a demodulation circuit to demodulate SSB (Single Side Band) signal, and to a carrier reproduction circuit for the demodulation circuit.
2. Description of the Related Art
In the field of radio communication, filters for canceling unwanted wave (noise) are used at each of the radio frequency (RF), intermediate frequency (IF), and audio frequency (AF) stages of the radio receiver. And for these filters, various improvements have been proposed.
For example, the applicant of the present application proposed "ADAPTIVE FILTER SYSTEM" disclosed in U.S. patent application Ser. No. 08/452,282 now U.S. Pat. No. 5,716,996, for canceling unwanted waves in the AF stage, and for outputting only an objective signal by setting the coefficients of the digital filter using an adaptive algorithm.
FIG. 1 shows the basic structure of the adaptive filter system disclosed in the U.S. patent application Ser. No. 08/452,282. The adaptive filter system is comprised of a delay circuit 91, a filter 92, a subtracter 93, and a controller 94. The delay circuit 91 delays an AF input signal and supplies the delayed signal to the filter 92, and the filtered signal output from the filter 92 is output as an AF output of the adaptive filter system. The subtracter 93 obtains the difference between the AF input signal and the AF output signal. The controller 94 controls the characteristic of the filter 92 based on the difference obtained by the subtracter 93, in accordance with LMS (Least Mean Square) algorithm.
This adaptive filter system is provided at the AF stage. Therefore, the circuits at the RF stage or the IF stage of, for example, a demodulation circuit process the signal including unwanted components. Therefore, the dynamic range of these circuits become narrow and processing error is large. Further, when AGC (Automatic Gain Control) is performed at the IF stage, the amplitude of the object (target) signal is also erroneously adjusted (suppressed) based on the level of the unwanted signal.
A BPF (Band Pass Filter) is used in the superheterodyne receiver to cancel unwanted reception signals. Transmitters for amateur and service radio can change the frequency of the transmission signal within an allowable range. Thus, there is a large possibility that an unwanted signal is in the reception band (in the passing band of the BPF) of the receiver. Further, image interference, radio frequency interference, spurious beats, or the like are easily contaminated to the reception signals. Notch filters are used for conventional amateur and service radio transceivers to cancel unwanted signals in the passing band of the BPF. In recent amateur and service radio transceivers, digital notch filters or adaptive digital notch filter systems are used whose filter coefficients are set by using adaptive algorithms.
An adaptive notch filter system, disclosed in U.S. Pat. No. 5,226,057, for example, converts a received radio-frequency signal to an intermediate-frequency signal, and supplies the intermediate-frequency signal to a plurality of notch filters connected in series to cancel unwanted signal components, then demodulates and outputs the signal output from the series-connected filters.
However, when the notch filter system is formed by using a digital signal processing technique, the processing amount becomes much and processing errors increases as the amount of processing increases.
Recently, in the field of mobile communications or the like, an RZ (Real Zero) SSB system has been proposed as a radio communication system having a good phasing property and a high frequency utilization factor.
The conventional demodulation circuit of the RZSSB signal comprises an FM demodulation circuit having a limiter, a frequency detection circuit, and an integration circuit, and a linearizer. The linearizer, however, comprises a Hilbert transformer, and the quality of the demodulation signal is greatly influenced by the characteristics of the Hilbert transformer. Therefore, although the Hilbert transformer having a good property is required, it is difficult to design the Hilbert transformer having a good property.
To solve this problem, the applicant of the present application also proposed demodulation circuits without a linearizer in Japanese Patent Application Hei 6-206907 and U.S. Pat. No. 5,548,243.
A carrier reproduction circuit disclosed in Japanese Patent Application Hei 6-206,907 produces a low-frequency signal by mixing the received intermediate frequency signal with a local oscillation signal, and abstracts a carrier by passing the low-frequency signal through an active bandpass filter with a narrow pass-band.
A carrier reproduction circuit disclosed in U.S. Pat. No. 5,548,243 prepares an intermediate frequency signal by mixing a received signal with a local oscillation signal, and abstracts a carrier by passing the intermediate frequency signal through the BPF. An FM detection circuit detects the signal output from the BPF. The carrier is reproduced by controlling the local oscillation signal using the detected output.
These carrier reproduction circuits have to reproduce not only the frequency but also the phase of the carrier exactly. When the carrier reproduction circuit comprises an analog circuit, however, the exact reproduction of the carrier is difficult. When the carrier reproduction circuit comprises a digital circuit, its structure and the processing it performs become complex.